No, you can't pump out native 15khz from Voodoo...would be great if you could though!
Reasons for Mediator...
1) Expansion possibilities into USB, Network, Soundcard, TVCard and more!
2) Perhaps beef it up with a BPPC and a copy of OS4BPPC when that comes
3) AGA is not meant for big monitors!
4) Powericons (ok lame one)